Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

What are you basing your calculations on? Are you not a TV repairman by trade?

Using the 35f temperature the OP stated in the original post and square footage. Of course a load calculation would yield more accurate results. I've seen way too many oversized furnaces, many of them cycling on high limit switch due to low airflow/poor ductwork/oversizing.

Got out of the TV business back when the prices of new TV's dropped to a point where there was no money to be made repairing them. Glad switched to HVAC, TV prices have just continued to plummet ever since I got out of the business. Saw an LG 60" flat screen for $800 recently, 42" TV's under $400.

Yes, 123,000 Btu is the same as 123k. It is not the input that matters, but rather the output. If your furnace is an 80% efficient furnace, it's output heating your house is 98,400 Btu, but if your furnace is 95% efficient, you have 116,850 Btu of heat being pumped into your house.

If you are located in a fairly northern part of the world, your furnace may be fine for your application.

On the data plate it says "Bonnet Capacity=98000". So, based on what you said above, it would seem that this furnace is an 80% unit, with 98,000 BTU going into the house?

On the data plate it says "Bonnet Capacity=98000". So, based on what you said above, it would seem that this furnace is an 80% unit, with 98,000 BTU going into the house?

Not really. Older furnaces, ones that do not use induced draft blowers, are rated differently. Bonnet capacity ratings are not actual efficiency ratings, but is the maximum amount of heat that furnace bonnet can handle. Most furnaces just prior to induced draft furnaces were in the 65-70 percent efficiency rating.

I just read over your original post for specifics about your area. The usual temperature range for the Tacoma, MD (District of Columbia area) is to provide 70 degrees indoor temperature when the outdoor ambient temps are at 17 degrees. Once again, your furnace may be fine for your area, but if replaced, should be of a lot less Btu input value.

In your area, if your home is comfortable with a 3.5 ton (42,000 Btu) cooling system, it will kept warm with less than double that amount of heat (84,000 Btu), which it is possible what your existing furnace is producing. If you currently have a 3.5 ton cooling system, I would recommend a 90k condensing furnace as a nice replacement. If your cooling system is a 3 ton, and that amount of cooling is keeping your house comfy, go with a 70k condensing furnace (it indicates your house is tightly built and decenlty insulated.)

RoBoTeq, you live closer to the OP than I do, interesting theory on sizing furnaces based how much AC it takes to make a house comfortable. Here in Oklahoma we're closer to a 1:1 ratio. Normally the blower size dictates the minimum furnace required, which usually exceeds the 1:1 ratio. Some contractors take it one step further and use the high BTU for a given size blower because that's what the supply house has in stock. The result is a furnace that rides the high limit because the contractor doesn't upsize the ductwork when they install the oversized furnace. Unless the customer has a system powered thermostat that goes blank when the system hits high limit the customer have no idea the furnace is cycling on high limit unless they happen to be watching the burners. As long as the house is warm and warm air is coming out of the vents you will rarely get a call. Since the furnace is typically 2X the size needed the house will stay warm even when the burners are shut off 1/2 the time on high limit.

John01 understand that steady state efficiency and AFUE are 2 different things. Your furnace could very well be 80% while running, but cycling losses and pilot light (if your uses one) reduce AFUE to 65-70%. Newer furnaces have much lower cycling losses and no pilot lights therefore the 80% Input/Output ratio is closer to AFUE.

The 123k input rating was when the furnace was new, it may or may not be that now. Ideally it should be the same now as when it was installed, but gas burner orifices can partially clog, and the gas valve can loose it's calibration. You can "clock your gas meter" and see how much gas it's actually burning.

RoBoTeq, you live closer to the OP than I do, interesting theory on sizing furnaces based how much AC it takes to make a house comfortable. Here in Oklahoma we're closer to a 1:1 ratio. Normally the blower size dictates the minimum furnace required, which usually exceeds the 1:1 ratio. Some contractors take it one step further and use the high BTU for a given size blower because that's what the supply house has in stock. The result is a furnace that rides the high limit because the contractor doesn't upsize the ductwork when they install the oversized furnace. Unless the customer has a system powered thermostat that goes blank when the system hits high limit the customer have no idea the furnace is cycling on high limit unless they happen to be watching the burners. As long as the house is warm and warm air is coming out of the vents you will rarely get a call. Since the furnace is typically 2X the size needed the house will stay warm even when the burners are shut off 1/2 the time on high limit.

Blower size has absolutely nothing to do with sizing HVAC systems for cooling or heating. Btu values in heat gain and heat loss are the only things that matter. In my area, it just so happens that if a certain amount of cooling will cool a house, just less than twice that amount of energy in heat will heat the same sized house. That is assuming there are no oddities such as extremely high ceilings throughout the house, massive amounts of windows or an indoor swimming pool or hot tub in the house.

Yes, it does have a pilot light. And, I don't have any cooling attached. This is just a furnace.

Now, I just did a seat of the pants check of furnace cycling, because I don't have a data logger to capture the data. I did not clock the meter, as suggested by 54regcab. Didn't think to do it.

The current outside temperature is 39 f.

The thermostat set temp is 71 f.

When it drops to 70, the furnace turns on. It runs for fourteen minutes, and raises the temperature to 72 f.

It takes an hour and 25 minutes for the inside house temp to drop from 72 to 70, and then the furnace turns on again.

Is that helpful information?

For your area...Tacoma, WA (which I was not aware existed), it sounds like your current furnace is properly sized, assuming that the outdoor temps do get colder than it is currently. I know that Washington state is not as cold as it is in the Northeast, but I am assuming the temps do drop below freezing that far north. Are my assumptions correct?

I don't know what clocking the meter would tell you other than how much gas you are using per hour, which is strictly input data that has no relevance to heating the house.

John01 understand that steady state efficiency and AFUE are 2 different things. Your furnace could very well be 80% while running, but cycling losses and pilot light (if your uses one) reduce AFUE to 65-70%. Newer furnaces have much lower cycling losses and no pilot lights therefore the 80% Input/Output ratio is closer to AFUE.

The 123k input rating was when the furnace was new, it may or may not be that now. Ideally it should be the same now as when it was installed, but gas burner orifices can partially clog, and the gas valve can loose it's calibration. You can "clock your gas meter" and see how much gas it's actually burning.

This information is innaccurate. Bonnet temperature has nothing to do with cycle rates and cycle rates have nothing to do with efficiencies.

For your area...Tacoma, WA (which I was not aware existed), it sounds like your current furnace is properly sized, assuming that the outdoor temps do get colder than it is currently. I know that Washington state is not as cold as it is in the Northeast, but I am assuming the temps do drop below freezing that far north. Are my assumptions correct?

I don't know what clocking the meter would tell you other than how much gas you are using per hour, which is strictly input data that has no relevance to heating the house.

The temps will get below freezing, but not often, and not for very long.

The temps will get below freezing, but not often, and not for very long.

Sounds like, even though you are further north than I am in PA, you don't get as cold weather as I do. So, definitely have a load calculation done before having a new furnace installed. You just may be fine with a 60-70K condensing furnace in the 90+ efficiency range.

The longer cycles will even out the air temperatures throughout your house and you overall feel more comfortable while saving money on energy.

Blower size has absolutely nothing to do with sizing HVAC systems for cooling or heating. Btu values in heat gain and heat loss are the only things that matter. In my area, it just so happens that if a certain amount of cooling will cool a house, just less than twice that amount of energy in heat will heat the same sized house. That is assuming there are no oddities such as extremely high ceilings throughout the house, massive amounts of windows or an indoor swimming pool or hot tub in the house.

In our area the amount of cooling required dictates the minimum blower size needed. For Oklahoma any furnace that has enough blower capacity for cooling will only come in a size that will provide sufficient heating. To get a 3 ton blower means buying a minimum 50K furnace, 4 ton 75k, 5 tons 100K, etc.